I actually own a Yakima SwingDaddy right now - how were you planning to modify it? I suppose you could drill a hole directly through the main vertical beam?I've romanced the notion that a used tip-down bike carrier from craigslist modified with a spare tire mount (such as for a trailer) would be a cheap, workable solution for an external spare carrier. Should be able to do it fot $100 or less. Perhaps someday I will try cobbling it together. Might be rickety and ugly, but I think it'd work well enough.